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What Is Body Mass Index?

The body mass index (BMI) is a measurement we often use to understand a person’s fatness and if they are at risk for heart disease. It is calculated using your height and weight. There are numerous online calculators that you can use to measure your BMI. A good site that is sponsored by the CDC can be found here. This website also provides helpful tips for weight management for both adults and children.

Why Do We Use the Body Mass Index?

Using your BMI you can determine your weight status. A BMI <19 is considered underweight, a BMI from 19-24 is normal, a BMI from 25-30 is overweight, and a BMI >30 is obese.

One of the biggest limitations of the BMI is that it does not take into consideration the unique characteristics of our bodies. For example, there are many things that affect our weight in addition to fat. These include our bone mass and frame as well as our muscle mass and density.

An example that illustrates this limitation is considering the potential health of two different men that are both 30 years of age:

One is 5 foot 10 inches tall and weighs 195 pounds. With these measurements his BMI is 28 and he is considered overweight. He admits to carrying around a few extra pounds and his waist is 39 inches.

Another man is 5 foot 10 inches tall and weighs 190 pounds. He exercises daily with a focus on weight lifting and resistance training. His BMI is 27.3 and as such is considered overweight by the same standard as the other man. His describes himself as fit and his waist is 32 inches.

This is a simple example that illustrates an important point, our health and risk of heart disease involves much more than our height, weight, or BMI.

So if the BMI measurement is not helpful, then you may be thinking “why do we use it all the time?”. First, it works well for the average person. This means that if you consider yourself average as far as your focus on health, exercise, diet, and personal habits, then the BMI may really provide information that actually reflects your health risks and fatness. Second, BMI is an easy calculation that can be used as a screening tool when we address public health needs.

Healthy Living Versus Body Mass Index

What if you consistently have a BMI >25 or perhaps >30 despite your efforts to remain active, exercise, and maintain a healthy diet? Are your efforts futile compared to those around you that are skinnier that may not pay as much attention to their health?

An interesting new study examines these very questions (1). The study was from Norway and called the Nord-Trøndelag health (HUNT) study. The study included 61,299 people that were studied on average over 12 years. Using BMI measurements and the definitions outlined previously the researchers concluded that 16.4% of the population were considered obese and another 43% overweight.

The authors then tried to determine if the study participants were metabolically unhealthy independent of their BMI measurements. Then they compared people from all BMI ranges that were metabolically unhealthy to those that were healthy with similar BMIs. People were categorized as metabolically unhealthy if they had a large waist circumference [men >37 inches (94 cm), women >31 inches (80 cm)] and two or more risk factors. These other risk factors included a high fasting triglycerides, low fasting HDL (good) cholesterol, an elevated blood pressure (=130/85 mm Hg) or use of blood pressure medications, diabetes, or an elevated non-fasting glucose (=200 mg/dL).

Over the study period 1,201 people developed heart failure and 2,547 suffered their first heart attack. The authors found in people with a BMI <25 (normal) that were metabolically unhealthy the risk of a heart attack was 90% higher compared to people with the same BMI that were metabolically healthy. The risk of heart failure was 10% higher. These risks persisted despite the level of activity in a week performed.

In all other BMI groups the risk was much higher in those metabolically unhealthy. For example, in those with a BMI from 25-30, there was a 70% increased risk of heart attack and 40% increased risk of heart failure if they were metabolically unhealthy. Finally, for those with a BMI of >30, there was a 100% increased risk of heart attack and 70% risk of heart failure. Again, these risks were seen regardless of the amount of physical activity per week.

Specifically looking at the risk of a heart attack, all BMI groups (<25, 25-30, >30) that were metabolically healthy did better than all BMI groups that were metabolically unhealthy. For example, an obese (>30 BMI) healthy patient’s risk of a heart attack was lower than a normal weight (<25 BMI) metabolically unhealthy patient. For heart failure risk, lower BMI patients did better in general, but being metabolically unhealthy increased risk across all BMI groups.

What Does This Study Teach Us About Heart Health?

This study teaches us several important things.

First, as mentioned above, our fitness and health is more than our height and weight. When my patients tell me that they know that they need to lose weight, I typically say that I want them to become more fit and healthy rather than focus on weight. This health focus includes increasing activity, exercise, and a healthy diet that lowers our cholesterol, glucose, and blood pressure. This healthy diet should also include minimizing salt intake. As mentioned in my prior posts, healthy lifestyles also need to include restorative sleep.

Next, regardless if you look thin, fat, or somewhere in between, you need to also focus on other potential risk factors. This includes a yearly physical examination with measurement of cholesterol, glucose, and blood pressure. I have many patients that are thin and on the outside look very healthy, but have severe heart disease because of longstanding high blood pressure or diabetes or high cholesterol or smoking habits. Finally, if you have a BMI >25, but you are active, try to be healthy, and don’t have other heart risk factors, you are on the right track and your long-term risk of heart disease is lower than those with risk factors even if they have a lower BMI.

CREDIT: Brian Jackson/Alamy

Last Updated:
1/16/2014

Important: The views and opinions expressed in this article are those of the author and not Everyday Health. See More

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